It is known that research and studies developed in the tire-planning field have aimed at simultaneously achieving many qualitative features, such as (only mentioning some of them selected from those of the greatest importance) roadholding, steering property, rolling resistance, comfort, high kilometric yield and good wear evenness and, above all in tires for motor-buses, trucks and transport vehicles in general, resistance to fatigue under heavy operating conditions, characterized by high vertical and transverse loads and high environmental temperatures.
The Applicant has already achieved a good compromise solution between said features, by a tire described in the Italian Patent No. 1125578 to which reference is herein made as the most pertinent state of the art. This radial tire has a usual carcass structure composed of cords extending in radial planes containing the rotation axis of the tire, to which a belt structure is applied which preferably comprises first and second radially superposed belt strips and a pair of side straps circumferentially applied to the opposite side edges of the radially external belt strip. Said belt strips are formed of metallic cords parallel to each other in each strip and oriented obliquely in respectively opposite directions with reference to the equatorial plane of the tire, and they have widths slightly differentiated from each other according to a given scaled down or consecutively staggered pattern.
In turn, the side straps are formed of a plurality of metallic cord coils (a continuous winding or several distinct coils) circumferentially disposed, so as to define two radially superposed layers of coils placed in axial side by side relation and oriented parallel to the equatorial plane of the tire.
One of the fundamental roles of the side straps is that of conveniently "belting" or confining the underlying belt strips at the edges of the strips themselves, which are the most critical areas with reference to the resistance to fatigue of the belt structure and therefore the tire as a whole. In fact, at such areas the ends of the individual metallic cords forming the belt strips have more freedom of movement than at the intermediate portions of the longitudinal extension of the cords themselves, that is at the central area of the tire crown portion.
This greater freedom of movement in the tires devoid of said straps or other confining elements results in great stresses applied to the ends of the belt structure and involving deformations, especially shearing stresses that will trigger tearings in the rubberizing blend and separation of the belt from the carcass ply and also between the strips of the belt structure.
For the purpose of obviating all the above drawbacks, at the present time positioning of the side straps is selected as much as possible close to the side edges of the belt structure, in an attempt to localize the confining action as much as possible in the vicinity of the edges. As it is not technically possible to make the external side edges of the straps coincide with the edges of the underlying belt strip, neither is it obviously possible to make them project therefrom. The technical solution hitherto adopted for achieving the above purpose has been that of giving the belt strips a consecutively scaled down or staggered pattern, so that the second strip is larger than the first strip placed adjacent to the carcass. Under this situation, the external edges of the side straps can be positioned a short distance away from the edges of the second belt strip so that they substantially correspond with the side edges of the first belt strip, thereby ensuring the necessary confining action on the belt strip edges.
Mainly for the above reason, the possibility of using an inversely staggered pattern, that is with the larger belt strip at a radially internal position, was unthinkable in heavy duty tires of the below described type. This is because it could be proved from experience that, in operation, the residual width of the unconfined edges of the belt structure was sufficient to trigger early and unacceptable separations of the belt structure members from each other and from the carcass, resulting in the tire becoming unusable.